Kenmore "Elite" Microwave Gliding Tray jumps Off Track

We purchased a Kenmore Elite Microwave Hood Combo in July 2007, Model # 721.80833500. It was manufactured in June 2006. The gliding tray and glass dish jump off the track. This problem began roughly 2 years after purchase. The microwave interior is clean and there are no visible reasons why this problem should occur. Based on other posts, apparently there is a manufacturing defect. When I called Sears to discuss the problem, their rep was only interested in selling us a "flat fee" of $129.00 for a service call. This after already planning to charge us $70.00 for a inspection call for our Kenmore "Elite" refrigerator, with the same tech! And, exclusive of any required parts! Though the idea of walking in to Sears and 'exchanging' the glass dish is amusing, is this part really the problem? The Sears service rep did not know. Shame on Sears for letting consumers bear the cost of their product design learning curve. I truly want to support our U.S. companies, but the lack of commitment to service and quality continue to erode my trust. Sorry you lost your HQ building in downtown Chicago.